1. Field of the Invention
The present invention relates to a CDMA mobile station apparatus for receiving a control spread signal and a spread data signal via a radio link, and more particularly, to a CDMA mobile station apparatus for canceling a control spread signal partly or wholly as an interference signal from a received signal so as to increase the accuracy with which to demodulate a data signal.
2. Description of the Related Art
CDMA (Code Division Multiple Access) systems are known as point-to-point connection systems based on spectrum spread communication principles. In a CDMA radio communication system, a signal that is spread by a spreading code is transmitted via a radio link between a base station apparatus, i.e., a CDMA base station apparatus, and a mobile station apparatus, i.e., a CDMA mobile station apparatus. In such a system, a plurality of signals can be spread by different respective spreading codes and the spread signals can be transmitted by way of multiplex communications.
Generally, the base station apparatus of the above described system, which performs cellular communications based on W-CDMA principles, transmits individual spread data signals to respective mobile station apparatus and a control spread signal to all of the mobile station apparatus that are parallel to each other via radio links. The mobile station apparatus respectively demodulate the individual spread data signals from the signals which have been received via the radio links.
The individual spread data signals that are transmitted to the respective mobile station apparatus comprise, for example, spread signals that are transmitted via dedicated physical data channels (DPDCH) for respective users, and the control spread signal that is transmitted to all of the mobile station apparatus comprises, for example, a spread signal that is transmitted via a broadcasting channel (BCCH) (hereinafter referred to as xe2x80x9cspread broadcasting signalxe2x80x9d) for all of the users or a spread signal transmitted via a common pilot channel (CPICH) (hereinafter referred to as xe2x80x9cspread common pilot signalxe2x80x9d)for all of the users, for example.
Conventional systems and their problems will be described below with respect to spread data signals for DPDCH as individual spread data signals and spread broadcasting signals for BCCH as control spread signals.
The spread data signal is produced by spreading a data signal with a spreading signal for data signals. The spread data signal has a variable information transmission rate (and spreading ratio) so as to be compatible with services of different information rates although the bandwidth (chip rate) of the spread data signal is constant. Since the required communication quality varies depending on the magnitude of the information transmission rate, the transmission power of the spread data signal is made variable so as to be able to meet variations in the communication quality.
The spread broadcasting signal is produced by spreading a broadcasting signal with a spreading signal for broadcasting signals. The spread broadcasting signal has a constant information transmission rate and a constant transmission power.
The spread data signal is used to transmit data individually from the base station apparatus to the mobile station apparatus, and the spread broadcasting signal is used to transmit common information from the base station apparatus to all of the mobile station apparatus. The spread broadcasting signal is also used for the mobile station apparatus to establish synchronism with the base station apparatus.
Specifically, when the mobile station apparatus are switched on, the mobile station apparatus use the spread broadcasting signal in an initial connection sequence, for connection to the base station apparatus. However, the mobile station apparatus do not use the spread broadcasting signal for individual data communications with the base station apparatus.
FIG. 19 of the accompanying drawings shows an example of the frame format of a perch channel (BCCH) used to transmit a spread broadcasting signal via a radio link. As shown in FIG. 19, one frame of the BCCH includes 16 slots #0-#15. Each of the slots #0-#15 includes an LC section 141 having 9 symbols #0-#8, for example, that are spread with a long code (LC) having a period of 10 msec. and a short code (SC) having a period of 62.5 xcexcsec., and a long code mask symbol 142 comprising one symbol, e.g., a symbol #9 at the final end of the slot spread by a short code.
The long code mask symbol 142 is composed of the sum of a short code (common short code) 143 that is shared by the system and a short code (inherent short code) 144 that differs from base station apparatus to base station apparatus or from sector to sector. The base station apparatus has a plurality of sectors.
The long code refers to a spreading code having a relatively long period, and the short code refers to a spreading code having a relatively short period. One symbol comprises 256 chips, for example.
An arrangement and operation of a receiver of a mobile station apparatus in the above system will be described below.
FIG. 20 of the accompanying drawings shows an example of the receiver of the mobile station apparatus. In the receiver, as shown in FIG. 20, an antenna 151 receives a signal that is transmitted from a base station apparatus via a radio link, and a reception unit (RX) 152 downconverts the received signal in a carrier frequency band into a baseband signal. A spreading code generator 153 is capable of generating a plurality of spreading codes for respective signals or channels that are desired to be received. The receiver selects one of the spreading codes that is generated by the spreading code generator 153 so as to select a signal or channel to be received.
Specifically, a spreading code that is generated by the spreading code generator 153 is outputted to a complex correlator 154. The complex correlator 154 acquires correlated values between the received signal outputted from the reception unit 152 and the spreading code outputted from the spreading code generator 153. The correlated values outputted from the complex correlator 154 are then demodulated by a demodulator 155, thereby decoding information of the signal or channel corresponding to the spreading code.
Thus, by applying the spreading code for the broadcasting signal to the complex correlator 154, the broadcasting signal is decoded. For example, when a spreading code for a data signal is applied to the complex correlator 154, the data signal is decoded, and the information of each code-divided signal or channel is decoded.
FIG. 21 of the accompanying drawings shows another example of the receiver of the mobile station apparatus. As shown in FIG. 21, the receiver has a BCCH demodulator 163 for demodulating a broadcasting signal or a control signal and a data demodulator 164 for demodulating a data signal, where the BCCH demodulator 163 and the data demodulator 164 comprise separate processors from one another.
Specifically, an antenna 161 receives a signal from a base station apparatus via a radio link, and the received signal is processed by a reception unit 162. A spread broadcasting signal that is included in the received signal is demodulated into a broadcasting signal by the BCCH demodulator 163, and a spread data signal that is included in the received signal is demodulated into a data signal by the data demodulator 164.
However, when the above CDMA mobile station apparatus demodulates a data signal from a signal that is received from the base station apparatus via a radio link, since a spread broadcasting signal that is included in the received signal acts as an interference signal for a spread data signal included in the received signal, the accuracy with which to demodulate the spread data signal into a data signal tends to be lowered. In particular, the LC section 141 shown in FIG. 19 is generated by using a long code that is in a code orthogonal relationship to the data signal spreading code, whereas the long code mask symbol 142 is generated by using a short code that is not in a code orthogonal relationship, i.e., non-orthogonal, to the data signal spreading code. Therefore, the interference cause by the long code mask symbol 142 is greater.
A specific example of the above-identified interference will be described below with reference to FIGS. 22(a) through 22(c) of the accompanying drawings.
FIG. 22(a) shows an example of a string of correlated peaks of a data signal that is produced by a mobile station apparatus when no spread broadcasting signal is present (an ideal case). FIG. 22(b) shows an example of correlated values between a spread broadcasting signal and a data signal spreading code, i.e., levels of interference with a data signal. As schematically shown in FIG. 22(c), since an actual mobile station apparatus obtains the sum of the string of correlated peaks shown in FIG. 22(a) and the levels of interference shown in FIG. 22(b), the broadcasting signal acts as an interference signal with respect to the data signal.
If, for example, a data signal spreading code in a system having a bandwidth of 4.096 MHz is composed of 128 chips, i.e., the data signal spreading code has an information transmission rate of 32 kHz, then the correlator integrates a received signal for the time of the 128 chips in obtaining correlated values between the received signal and a data signal spreading code. In this case, the interference level of a spread broadcasting signal included in the received signal is reduced to a level of {fraction (1/128)}, which is relatively small as compared with the interference level before the received signal is despread, because of the integration for the time of the 128 chips (the integration corresponds to a low-pass filter (LPF)).
According to the CDMA scheme, the receiver is arranged so as to be able to increase the information transmission rate. Since the bandwidth is constant, if the information transmission rate becomes higher, then the integration time in the correlator becomes shorter. For example, if a data signal spreading code is composed of 16 chips, i.e., the data signal spreading code has an information transmission rate of 256 kHz, then the correlator integrates a received signal for the time of the 16 chips in obtaining correlated values between the received signal and a data signal spreading code. In this case, the interference level of a spread broadcasting signal included in the received signal is only reduced to a level of {fraction (1/16)} as compared with the interference level before the received signal is despread.
In general, a spread broadcasting signal needs to be transmitted from a-base station apparatus to all of the mobile station apparatus that are present in the communication range of the base station apparatus, i.e., a cell area that is covered by the base station apparatus. Therefore, it is customary for the base station apparatus to transmit the spread broadcasting signal on a radio wave at a power level that is large enough to enable the radio wave to reach the end of the communication range. If the interference level is reduced to only {fraction (1/16)}, then the interference of the spread broadcasting signal with the spread data signal is so large that it cannot be ignored, where such large interference results in the accuracy with which the data signal is demodulated, i.e., the reception quality of the data signal, being greatly deteriorated. The accuracy deterioration is caused due to the fact that whereas the level of the correlated peaks shown in FIG. 22(a) increases depending on the length of the data signal spreading code, i.e., the number of chips, the interference level shown in FIG. 22(b) is substantially the same regardless of the length of the data signal spreading code.
Generally, the above-described radio communication system is used in a multipath environment where a signal that is transmitted from a transmission apparatus, e.g., a base station apparatus, passes through a plurality of paths to a reception apparatus, e.g., a mobile station apparatus. Due to interference between multipath signals, the LC section 141 shown in FIG. 19 acts as a large interference signal with respect to a spread data signal. In the multipath environment, an entire spread broadcasting signal which generally has a larger transmission power than a spread data signal tends to act as an interference signal with respect to the spread data signal.
While conventional systems and their problems have been described above with respect to spread broadcasting signals, for example, similar problems occur with respect to spread control signals.
Specifically, the spread common pilot signal referred to above is produced by spreading a common pilot signal with a spreading code therefor, and the spread common pilot signal is a signal that is sent at a fixed rate to nonspecific users. The spreading code for the common pilot signal is in a code having an orthogonal relationship to a data signal spreading code. In the multipath environment, the entire spread common pilot signal tends to interfere with the spread data signal. The common pilot signal comprises a predetermined string of symbols, for example, and is used as interpolating information, e.g., phase information or the like, for each mobile station apparatus so as to perform coherent detection with respect to the base station apparatus.
It is therefore an object of the present invention to provide a CDMA mobile station apparatus which is capable of increasing the accuracy with which to demodulate a data signal when receiving a spread control signal and a spread data signal from a base station apparatus via a radio link.
To achieve the above-described object, there is provided, in accordance with the present invention, a CDMA mobile station apparatus for receiving a spread control signal including a non-orthogonal spread signal that is generated by using a non-orthogonal spreading code that is non-orthogonal with a data signal spreading code and a spread data signal that is produced by spreading a data signal with a data signal spreading code from a base station apparatus via a radio link. The CDMA apparatus according to the present invention comprises non-orthogonal spread signal generating means for generating a non-orthogonal spread signal by using a non-orthogonal spreading code, subtracting means for subtracting the generated non-orthogonal spread signal from a received signal so as to generate a difference signal, and data signal demodulating means for demodulating the difference signal into a data signal by using the data signal spreading code.
Since the non-orthogonal spread signal that is generated by using the non-orthogonal spreading code, which is non-orthogonal with the data signal spreading code, is canceled as an interference signal from the received signal, the accuracy with which to demodulate the received signal into the data signal is increased. Specifically, a larger interference is produced between two signals that are generated by using spreading codes that are non-orthogonal with each other than two signals that are generated by using spreading codes that are in code orthogonal relationship to each other. According to the present invention, at least interference between two signals that are generated by using spreading codes that are non-orthogonal with each other is canceled so as to increase the quality of the data signal that is received.
Furthermore, in view of the fact that the non-orthogonal spread signal that is included in the received signal is generally subjected to a phase rotation which is caused by a radio transmission path, the non-orthogonal spread signal generating means generates a non-orthogonal spread signal to which a phase rotation corresponding to the phase rotation has been imparted.
Therefore, the non-orthogonal spread signal that is subjected to the phase rotation that is caused by the radio transmission path is compensated for so as to cancel the non-orthogonal spread signal from the received signal. As a result, the accuracy with which the received signal is demodulated into the data signal is increased.
If the non-orthogonal spread signal that is included in the received signal is limited in band, then the non-orthogonal spread signal generating means generates a non-orthogonal spread signal which has been limited in band.
Therefore, the band limitation that is applied to the non-orthogonal spread signal by a transmission apparatus, i.e., the base station apparatus, or a reception apparatus, i.e., the CDMA mobile station apparatus, is compensated for so as to cancel the non-orthogonal spread signal from the received signal. As a result, the accuracy with which the received signal is demodulated into the data signal is increased.
Moreover, if the non-orthogonal spread signal that is generated by the non-orthogonal spread signal generating means has an error with respect to the non-orthogonal spread signal that is included in the received signal, then the non-orthogonal spread signal generating means generates a non-orthogonal spread signal whose signal intensity has been suppressed.
Consequently, even if the generated non-orthogonal spread signal suffers an error due to an estimated transmission path error, the signal intensity of the non-orthogonal spread signal is suppressed and canceled from the, received signal. As a result, the accuracy with which the received signal is demodulated into the data signal is increased.
According to the present invention, there is also provided a CDMA mobile station apparatus for receiving a spread control signal that is produced by spreading a control signal with a control signal spreading code and a spread data signal that is produced by spreading a data signal with a data signal spreading code from a base station apparatus via a radio link. This CDMA mobile station apparatus according to the present invention comprises: spread control signal generating means for demodulating a received signal into a control signal by using a control signal spreading code, and for spreading the demodulated control signal with a control signal spreading code so as to generate a spread control signal; subtracting means for subtracting the generated spread control signal from the received signal so as to generate a difference signal; and data demodulating means for demodulating the difference signal into a data signal by using a data signal spreading code.
Since the spread control signal that acts as an interference signal with respect to the spread data signal is canceled in its entirety from the received signal, the accuracy with which to demodulate the received signal into the data signal is increased. It is particularly effective to cancel the entire spread control signal if the CDMA mobile station apparatus is used in a multipath environment.
In view of the fact that the spread control signal that is included in the received signal is generally subjected to a phase rotation that is caused by a radio transmission path, the spread control signal generating means generates a spread control signal to which a phase rotation corresponding to the phase rotation that is caused by the radio transmission path has been imparted.
If the spread control signal that is included in the received signal is limited in band, then the spread control signal generating means generates a spread control signal which has been limited in band.
Moreover, if the spread control signal that is generated by the spread control signal generating means has an error with respect to the spread control signal that is included in the received signal, then the spread control signal generating means generates a spread control signal whose signal intensity has been suppressed.
The data signal or spread data signal referred to herein comprises a data communication signal including character data, image data, audio data, etc. that is to be transmitted between the base station apparatus and the CDMA mobile station apparatus.
The control signal or spread control signal referred to herein comprises a control signal which represents control information for establishing synchronism between a base station apparatus and the CDMA mobile station apparatus, and includes control data for achieving a desired control process, for example.
The data signal or spread data signal may be a DPDCH signal, and the control signal or spread control signal may be a broadcasting signal or a spread broadcasting signal or a common pilot signal or a spread common pilot signal. Alternatively, the data signal or spread data signal or the control signal or spread control signal may be another signal. Inasmuch as the CDMA mobile station apparatus of the present invention increases the accuracy with which the received signal is demodulated into the data signal by canceling, partly or wholly, a signal other than the data signal or spread data signal and tends to interfere with the data signal or spread data signal, from the received signal, the principles of the present invention are also applicable to such an interference signal by regarding the interference signal as the data signal, the spread data signal, the control signal, or the spread control signal.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of examples.